This invention relates to lasers and more particularly to lasers utilizing phase conjugate reflectors. In addition, this invention relates to high energy lasers or laser systems having high volume amplifier stages with high average power output.
Many potential laser applications such as laser communications, fusion, and drivers for nonlinear harmonic convertors and Raman devices require the use of very high energy laser sources exhibiting high brightness. High brightness can be generally defined as high power per unit area per unit bandwidth per unit solid angle. However, high brightness output from lasers depends on many factors that are either hard to control or have known operational limits. This results in lasers that are unable to provide the desired brightness output using current methods and apparatus.
Possible limiting factors such as optical path length variations, optical element quality, reflective coating breakdown, or optical aberrations have been addressed by a variety of techniques. Among these are refined manufacturing techniques for media and optics, and employing deformable or phase conjugate mirrors. Examples of the latter two techniques are described in U.S. Pat. No. 3,967,899 issued to T. O'Meara and U.S. Pat. Nos. 4,233,571 issued to Wang et al. and 4,321,550 issued to Evtuhov. These techniques, among other things, confine radiation to preferred operating modes, and decrease degradation due to a variety of optical aberrations. However, these techniques do not address several problems associated with laser energy scaling.
One major problem with increasing the energy available from lasers is the fact that there is an inherent limit to the energy per unit volume that can be extracted from or stored in a lasing medium. Therefore, for high energy applications the volume of the laser must be increased in order to increase the energy available for output. Unfortunately there are limitations on simply increasing the laser medium volume.
In chemical lasers the medium can often exhibit thermal gradients or material flow problems leading to undesirable chemical reactions or breakdown and depletion zones. These problems are increasingly difficult to prevent for larger media volumes. Safety problems also arise for chemical media that require specialized packaging or cooling in higher volumes.
Solid-state or crystal-based lasers have limits to the size of media that can be manufactured. Some crystalline materials can not yet be grown in large volume boules. Materials such as Nd:YAG have inherent constraints that impose a size limit on the order of 12 mm in cross-section (diameter) before factors such as impurities or stress during growth degrade the medium to an unsatisfactory or unusable form. This size constraint limits the energy output from Nd:YAG to approximately 2 joules which is unacceptably low for many high brightness applications. In addition, crystalline materials may grow too slowly to provide practical production rates for manufacturing large volume laser media.
Even when larger media volumes are possible, the energy storage is limited by parasitic modes, and if these can be eliminated, ultimately always by amplified spontaneous emission (ASE). Parasitic modes or oscillations are supported by reflections from the surfaces of the media or nearby optics. Larger volumes allow more gain length for alternative optical modes within the lasing medium, decreasing the energy available for extraction by the desired primary, "power", mode.
Even if stray reflections and parasitic modes can be eliminated, the increased volume provides greater gain length for spontaneous emission. This leads to depletion of energy for the desired laser transitions. In addition, while ASE can be measured in a given volume it does not straight forwardly or directly scale with increasing volume. The inability to accurately infer an ASE limit from a small volume to larger volumes makes accurate prediction of large volume performance very difficult or imprecise.
While parasitic modes and spontaneous emission may either be partially controlled by various absorbing filters and judicious choices of geometry or polarizers, or possibly so weak as to be ignored for low level, small volume laser operation, it is clear they represent the fundamental limiting factor in high energy laser applications.
What is needed is a method or apparatus for increasing the volume of laser media in a given laser system without increasing the presence of degrading factors nor reaching inherent volumetric limitations.
One approach might be to position several lasers in parallel and sum the outputs in order to provide an overall higher energy beam. This approach produces an incoherent sum of the individual outputs, such that, in the best case, the total far field brightness of N parallel laser outputs is N times the brightness of a single output. On the other hand, if the individual laser outputs can be arranged to have a fixed phase relationship with one another, a coherent sum is produced in which case it is possible for the peak far field brightness to increase proportional to N.sup.2. This highly desired situation can, in principle, be accomplished using one of two techniques.
First, there is the technique of evanescent coupling. This has been primarily applied in the area of diode lasers, although some lasers utilizing a fluid medium may have also incorporated this approach. In this technique, evanescent fields from adjacent laser channels interact to phase-lock the laser outputs from adjacent laser apertures. This is described in more detail in "Phased Array Diode Lasers" by W. Streifer, R. D. Burnham, T. L. Paoli and D. R. Scifres in Laser focus/Electro-Optics, June 1984. However, even though coupled through the evanescent fields, the output from adjacent laser channels may not be exactly in phase; adjacent channels usually operate in modes that are 180 degrees out of phase, producing a two-lobed far-field intensity distribution which is usually highly undesirable in device applications. The greatest brightness is achieved for parallel lasers that are coherently coupled with zero phase difference.
An alternate approach is to insert an electro-optical material in the optical path between each laser medium and the system output aperture. Applying a controlled voltage to the electro-optical material allows the alteration of the index of refraction and subsequently the speed of optical radiation traversing the material. This allows the adjustment of the phase of each laser relative to the others. However, the relative phase of a given laser output is dynamic; it changes over time due to factors affecting the laser medium optical path length such as temperature, stress, etc. In addition, one laser may need servicing or replacement and it is impossible to match exactly the old physical characteristics with the new laser because of inevitable physical length differences and compositional variations. Therefore, this technique would need to be dynamically adjustable to some minimal extent. This requires the use of specialized sensors, controls, electronics and programming. Such complexity is both impractical and unreliable for many applications.
What is needed then is a method of combining the output of multiple laser gain elements to achieve higher brightness, high energy output without volumetric limitations, and to provide coherent coupling.